Clients counting on big 401(k) earnings in retirement may need a reality check.

Reason: Many investors don’t factor in that long-delayed tax hit on their 401(k) once they start withdrawing money from it.

The average investor looks at the total value of their plans and makes budgeting decisions for retirement based on that.

Figuring out the tax payments on a 401(k) that’s matured isn’t tough for a financial advisor. But unless you have a crystal ball, estimating another likely tax hit awaiting older clients isn’t so easy.

Namely: Capital gains tax rates will likely go up, perhaps as soon as 2013, from the current 15% rate for taxpayers in the 25% bracket.

The selling point for a 401(k) was always that retirees enjoy a lower tax bracket than they do during their prime earning years. That may not be the case for the Baby Boomer wave of retirees.

Clients with some retirement savings are still ahead of the game, especially compared with those counting on Social Security as their main source of income.

But they’d better count on Uncle Sam taking a bigger slice of their nest eggs than in previous generations.

Are your retirement-age clients worried these days? Share your experiences below.

Tags: 401k, IRS, retirement planning, tax rates